It is often thought that our own modern age is unique in having a large number of people who live into old age. It has frequently been assumed that plagues, wars, and harsh working conditions killed off most people in previous ages before they could reach old age. However, recent research shows that in 17th century Europe, for example, people over sixty comprised 10 percent of the population. The studies also revealed that although infant mortality remained high until the 20th century in Europe, people who survived to adulthood could expect to live to be old.

The portions in boldface play which of the following roles in the argument above?